Legality of Trump's Immigration Ban

- bans, for 90 days, all immigrants (except green card holders) from seven majority-Muslim countries: Iran, Iraq, Syria, Libya, Sudan, Somalia and Yemen.
- instructs his administration to develop "extreme vetting" measures for immigrants from those seven countries to keep "radical Islamic terrorists" out of the United States.
- suspends the U.S. refugee program for 120 days.
- bars all Syrian refugees from entering the U.S. indefinitely.
- gives preference in admission to Christians, as the persecuted in majority-Muslim countries.

Why Were Those Seven Countries Singled Out?

Syria, Iran and Sudan are on the State Department’s list of state sponsors of terrorism since 1979, '84 & '93 respectively. Iraq, Libya, Somalia and are designated “terrorist safe havens” by the State Department.

Are Terrorists from Those Countries Dangerous?

Not really. They have injured a few people and were implicated in some terrorist plots. However, nationals of the seven countries have killed zero people in terrorist attacks on U.S. soil between 1975 and 2015. During that period, top terrorist killers were from Saudi Arabia - 2,369 murders, UAE - 314, Egypt - 162.

What Is the Legal Basis for the Order?

Section 212(f) of the Immigration and Nationality Act (INA) gives the President broad powers to ban any class of aliens from entering the country:

"Whenever the President finds that the entry of any aliens or of any class of aliens into the United States would be detrimental to the interests of the United States, he may by proclamation and for such period as he shall deem necessary, suspend the entry of all aliens or any class of aliens as immigrants or nonimmigrants, or impose on the entry of aliens any restrictions he may deem to be appropriate." [emphasis added]

That provision has been part of the INA since its enactment in 1952. Whenever the President determines that any class of aliens are “detrimental to the interests of the United States,” the President can ban their entry. So far there has been no case law imposing any firm legal limits upon that broad power (e.g., how wide can this restricted class possibly be, what factors should be considered "detrimental," how long should the ban last, etc.).

Have Other Presidents Banned Immigrants Under Section 212(F)?

Yes, but not as broadly as Trump. President Obama has issued at least 19 executive orders suspending the entry of aliens who are determined to have “contributed to the situation” in Iran, Syria, Libya, Burundi, Venezuela, Ukraine, South Sudan, engaged in certain transactions in North Korea, malicious cyber activities, human rights abuses, etc. George W. Bush has issued 6 such executive orders. Bill Clinton - 12.

What's the Difference Between Immigrants, Nonimmigrants and Refugees?

Terms "immigrants," "nonimmigrants" and "refugees" cover most foreign travelers. An immigrant is someone who chooses to resettle to the US. If they do it the legal way, they seek green cards and eventually can become citizens. Some immigrants, however, circumvent legal procedures and become undocumented illegal immigrants. On the other hand, a legal immigrant is a lawful permanent resident, a green card holder. They can keep renewing their green cards for life (as long as they don’t commit crimes) and have passed multiple screening processes.

A nonimmigrant is someone with a temporary visa. Refugees are those immigrants who seek asylum from war, persecution and other risks to their safety.

Does the Ban Apply to Green Card Holders?

Not in full force. Not if they pass other security checks anyway. Initially there was confusion over the green card holders. The reason there was confusion is because, on its face, the executive order applies to all "immigrants" (undocumented ones and green card holders as well - see the definition in the answer above). It was not clear whether the order is meant to ban existing legal immigrants from entry, or place on hold the procedures for the admission of new immigrants only As a result, green card holders were detained at the airports.

However, it has since been clarified in a statement by the Secretary of Homeland Security that “In applying the provisions of the president's executive order, I hereby deem the entry of lawful permanent residents to be in the national interest,” on a case-by-case basis, if they pass security checks. In other words, there is a presumption in favor of admitting green card holders but they can still be denied after a case-by-case review. The DHS later issued another clarification that states "we expect swift entry for these individuals."

Can Any Exceptions Be Made?

Yes. The Secretaries of State and Homeland Security will be able to jointly make a determination to admit individuals to the United States as refugees on a case-by-case basis if it is deemed to be in the national interest.

Does This Order Single Out Muslims?

Technically, no. The order does not specifically ban Muslims from any country, whether it is listed in the seven nations of particular concern or not. Non-Muslims, including Christians, are also included in the temporary ban from those countries.

But, even though it is technically not a Muslim ban, it will be publicly perceived as such anyway. That's because it applies to majority-Muslim countries only and, in 2015, Trump has issued a statement calling for a "total and complete shutdown of Muslims entering the United States."

Where Does It Say Anything About Christians?

The words "Christians" and "Muslims" do not appear anywhere in the text of the order. But Section 5(b) prescribes to "prioritize refugee claims made by individuals on the basis of religious-based persecution, provided that the religion of the individual is a minority religion in the individual's country of nationality." The seven countries affected by this order are all majority-Muslim. Trump has stated an intent to prioritize Christians in an interview with the Christian Broadcasting Network.

Is It Legal for the President to Prioritize Religious Minorities over Other Refugees?

So far, it has been legal, but the legality of the current broad policy will now be challenged like never before. There is already a policy in place that prioritizes refugees from religious minorities. The Lautenberg Amendment, first enacted in 1990 to facilitate resettlement of Jews from the former Soviet Union. The Amendment has since been expanded to include persecuted religious minorities in other countries, such as Jews, Christians, and Baha’is from Iran. The Amendment expires each year, which is why it has been reauthorized numerous times.
Nevertheless, Trump's call for giving preference to Christian refugees in this broad ban will now be challenged as violating the First Amendment’s Establishment Clause which prohibits giving official preference of one religion over another. At this point, it is difficult to predict the likelihood of success of that challenge.

What Happens After the Order Expires?

Anything can happen. This ordeal can be over, but that outcome is unlikely given the President's strong stance to tighten U.S. borders. There could be more countries on the list, more "extreme vetting" and other extreme measures forthcoming:

The Secretary of Homeland Security, in consultation with the Secretary of State, shall submit to the President a list of countries recommended for inclusion on a Presidential proclamation that would prohibit the entry of foreign nationals.

Haven't Some Federal Judges Already Banned the Trump's Order?

No, federal judges (three Democrats and one Republican) have just issued temporary "stays." They ordered the Customs and Border Protection (CBP), a division of DHS, not to remove anyone who has arrived in or was en route to the U.S. However, it's just a temporary order, a form of preliminary injunction which is limited in scope. It doesn't mean that the people detained at the airports are free to enter the U.S., it just means they cannot be deported right away. These orders will only last about a week or two. Two of these orders will expire, if not extended, on Saturday.

After that, the losing party will almost certainly appeal to the federal appellate courts. which have the power to set legally binding precedent within their respective federal districts that cover millions of people in the US. The appellate court decisions can then be appealed to the U.S. Supreme Court but it is under no obligation to accept any appeals. SCOTUS accepts appeals if the issue is of utmost importance, usually to reconcile the different rulings of different appellate courts on the same issue. It can take years for the case to be heard and ruled on in SCOTUS but it can hear a case much quicker if it decides to do so at its own discretion.

So, Is the Broad Trump Ban Legal, Overall?

What’s legal or not will ultimately be decided by the federal appellate courts (within their jurisdictions) and, possibly, SCOTUS. The legality of Trump's order won't be clear until those higher courts start ruling, after hearing the arguments of Trump's Department of Justice vs. civil rights and immigration advocacy groups.

They have a lot of novel issues to rule on. The President does have significant powers to control immigration, but those are constrained by the Constitution (Freedom of Religion, Due Process, Equal Protection), Congressional intent, laws that prohibit discrimination based on national origin and religion. E.g., 8 U.S. Code 1152 states that “no person shall… be discriminated against in the issuance of an immigrant visa because of the person’s race, sex, nationality, place of birth, or place of residence.” Note that this provision only applies to immigrants (green card holders).

So far, there has been very limited case law addressing exercises of presidential authority under Section 212(f). The most relevant case, perhaps, is the U.S. Supreme Court’s 1993 decision in Sale v. Haitian Centers Council, Inc. The Court upheld the U.S. executive order to intercept persons fleeing Haiti outside U.S. territorial waters and return them to their home country without allowing them to file for asylum and withholding of removal. That executive order was issued, in part, under the Section 212(f) of the INA.

Overall, SCOTUS has not been very generous to aliens over the years. However, they may have better luck at the lower federal appellate courts. The four federal district judges that have issued stays of the executive order did so because there was a basis for them to believe that plaintiff aliens had a substantial likelihood of prevailing at the higher courts.